1. Field of the Invention
The present invention relates to a stabilized foam. More particularly, the present invention relates to a stabilized foam suitable for use in medical and skin contact applications, having a crosslinked closed cell polyolefin foam, at least one stabilizing layer and at least one adhesive layer.
2. Description of the Related Art
For a variety of medical and skin contact products including first-aid bandages, tapes, absorbent dressings, wound covers and closures, scar reduction therapy pads, electrode grounding pads and drug delivery patches, foam is used as the exterior layer away from the skin. A skin contact adhesive is applied to this foam to provide adhesion to the skin. Many adhesives are in use and include acrylics in hot melt, solvent solution or aqueous suspension, synthetic block copolymer and other hot melts, silicone gels and silicone from solvent or hot melt systems. Often such foam films are white, tan or sheer in color and are frequently perforated for air breathability. They can also be printed with a design or logo.
An alternative unstabilized polyolefin foam material is presently in limited use. Other existing foams generally used in this application are typically vinyl or PVC based and when manufactured are cast on a release liner. This release liner provides stability to the foam while in the adhesive coating operation and possibly subsequent converting operations. The release liner can also be embossed which imparts the pattern to the foam being cast upon it.
There are several disadvantages to the use of PVC foams. PVC or vinyl foams have a plasticizer content that is inherent to the manufacturing process of PVC foam. These plasticizers have been investigated for possible medical safety risk with a concern of migration from the foam into the skin. The plasticizers in PVC also can migrate from the foam into the adjacent adhesive and/or release liners. There are regulatory limits to the use of PVC in Japan; similar restrictions are reportedly under consideration in Europe. PVC foams and films are under elevated safety scrutiny and medical device designers are receptive to alternative products.
Additionally, when bandages and tapes utilizing PVC foam are sterilized or simply aged, shrinkage of the foam is often observed. This shrinkage can be minor and is generally considered to be only a visual problem. But it can also be a more severe problem that can cause finished product rejections and/or can cause a visible exposed adhesive edge resulting from the PVC foam shrinking while the adhesive layer remains stable. The larger the die cut product, the larger the shrinkage is. Shrinkage in the PVC foam can also cause curling or rolling of the finished product. Shrinkage of any degree is not desired.
The plasticizer used in the PVC can also limit the selection of fillers and “actives” that could potentially be incorporated into or on the foam. Additionally, via a process known as “plasticizer migration” the PVC plasticizer can flow to adjacent materials and effectively limits what materials and adhesives can be adjacent to it. A typical problem is softening and expansion, even wrinkling, of an adjacent layer. Conversely, the PVC foam can absorb and “wick” oils and plasticizers into it from certain adhesives, especially hot melt adhesives containing oils. When this occurs, the PVC foam grows and changes shape over time.
Polyolefin (PO) foam is an alternative to PVC foam. This material is relatively low cost and can be manufactured in large volumes. However, it also has several undesirable properties. Polyolefin foams tend to be stiff and rigid when flexed and are easily torn or ripped. The polymers used in PO foams are cross-linked via application of electron-beam (e-beam) energy during manufacture. It is believed that this cross-linking leads to the stiffness and tearing tendency. In summary, if polyolefin foam is made to be thin and conformable, it is very weak and tears. Conversely, if thickness or density is increased to decrease tearing, the foam is stiff and not conformable to the skin.
Stiffness is generally not a desired property for foam used in a skin contact application. Flexibility and conformability are desirable properties to facilitate movements, such as, the flexing of finger, wrist, or elbow joints. Thus, despite the presence of an adhesive layer, stiff foams do not remain attached during flexing and, as a result, tends to lift off the skin.
Resistance to tearing is also critical. Of particular concern is the tearing when the foam is stressed in the cross-machine (CD) direction. Through polymer orientation in the manufacturing process, strength is concentrated in the machine direction (MD). Although strong in the machine-direction, so is the tendency to tear. For many potential end-use applications, this pronounced tear is a problem. Ideally, balanced stretch and tear properties are desired. First-aid bandages, tapes, wound covers or closures are not physically large, often ¾ inch to 2 inch at their narrow dimension. When the product is applied a degree of tension is required to attach and adhere it in place. This tension promotes tearing in PO foams. Often the PO foam is perforated to allow breathability. The physical apertures of the perforation imparted for breathability additionally weaken the foam and further promote tearing.
The low level of dimensional stability makes PO foams difficult to process through converting operations. Tensions within the unwinding pull-roll and die cutting zones result in elongation often with delayed recovery or no recovery. This tension and stretch imparted in the foam will often relax after processing and result in differential shrinkage of components and/or curling of the finished products. As thinner grades of PO foam are converted, the stretch and curling problems become exaggerated.
PVC foam has an appealing hand, good conformability and stretch recovery characteristics that make it suitable for bandages. Both PVC and polyolefin foams are economical products to manufacture.
Thus, there is a need in industry to identify alternatives to PVC and polyolefin foams that overcome these problems.
There have been efforts in recent years to locate an alternative to PVC foam to resolve the plasticizer and shrinkage problems without sacrificing the low cost or the appealing stretch/recovery properties. Alternate foams, such as, polyvinyl alcohol (PVA) foam, polyurethane foam, polyolefin foam, and the like, have also been evaluated. However, there have been no suitable alternatives found to date that meet all the design criteria listed above. The reasons for the absence of suitable alternatives include the following:
(1) polyvinyl alcohol foams are difficult to manufacture and not readily commercially available;
(2) polyurethane foams offer good physical properties but are very expensive;
(3) silicone foams also offer good properties but are more costly than the polyurethanes; and
(4) although polyolefin foams are relatively low cost, they have poor dimensional stability, exhibit stretching especially when thin and tend to rip or tear when stressed, particularly when a bandage is removed from a wound. Thus, because of their stiffness, polyolefin foams generally have poor adhesion to the skin and are not conformable to the skin when flexed.
The stabilized polyolefin foam according to the present invention provides an attractive and economical alternative to PVC and other foams.